[Clang] Do not allow [[clang::lifetimebound]] on explicit object member functions (#96113)

Previously, `[[clang::lifetimebound]]` applied to an explicit object
member function did nothing and was silently ignored.

Now issue the error diagnostic `'lifetimebound' attribute cannot be
applied; explicit object member function has no implicit object
parameter`

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -193,6 +193,9 @@ Attribute Changes in Clang
 - The ``hybrid_patchable`` attribute is now supported on ARM64EC targets. It can be used to specify
   that a function requires an additional x86-64 thunk, which may be patched at runtime.
 
+- ``[[clang::lifetimebound]]`` is now explicitly disallowed on explicit object member functions
+  where they were previously silently ignored.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10066,8 +10066,9 @@ def warn_null_ret : Warning<
   InGroup<NonNull>;
 
 def err_lifetimebound_no_object_param : Error<
-  "'lifetimebound' attribute cannot be applied; %select{static |non-}0member "
-  "function has no implicit object parameter">;
+  "'lifetimebound' attribute cannot be applied; "
+  "%select{non-|static |explicit object }0"
+  "member function has no implicit object parameter">;
 def err_lifetimebound_ctor_dtor : Error<
   "'lifetimebound' attribute cannot be applied to a "
   "%select{constructor|destructor}0">;

clang/lib/Sema/SemaDecl.cpp

@@ -6939,9 +6939,16 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
       // by applying it to the function type.
       if (const auto *A = ATL.getAttrAs<LifetimeBoundAttr>()) {
         const auto *MD = dyn_cast<CXXMethodDecl>(FD);
-        if (!MD || MD->isStatic()) {
+        int NoImplicitObjectError = -1;
+        if (!MD)
+          NoImplicitObjectError = 0;
+        else if (MD->isStatic())
+          NoImplicitObjectError = 1;
+        else if (MD->isExplicitObjectMemberFunction())
+          NoImplicitObjectError = 2;
+        if (NoImplicitObjectError != -1) {
           S.Diag(A->getLocation(), diag::err_lifetimebound_no_object_param)
-              << !MD << A->getRange();
+              << NoImplicitObjectError << A->getRange();
         } else if (isa<CXXConstructorDecl>(MD) || isa<CXXDestructorDecl>(MD)) {
           S.Diag(A->getLocation(), diag::err_lifetimebound_ctor_dtor)
               << isa<CXXDestructorDecl>(MD) << A->getRange();

clang/test/SemaCXX/attr-lifetimebound.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++2a -verify %s
+// RUN: %clang_cc1 -std=c++23 -verify %s
 
 namespace usage_invalid {
   // FIXME: Should we diagnose a void return type?
@@ -9,6 +9,7 @@ namespace usage_invalid {
     A() [[clang::lifetimebound]]; // expected-error {{cannot be applied to a constructor}}
     ~A() [[clang::lifetimebound]]; // expected-error {{cannot be applied to a destructor}}
     static int *static_class_member() [[clang::lifetimebound]]; // expected-error {{static member function has no implicit object parameter}}
+    int *explicit_object(this A&) [[clang::lifetimebound]]; // expected-error {{explicit object member function has no implicit object parameter}}
     int not_function [[clang::lifetimebound]]; // expected-error {{only applies to parameters and implicit object parameters}}
     int [[clang::lifetimebound]] also_not_function; // expected-error {{cannot be applied to types}}
   };